The invention relates to the treatment of loose material with liquid and/or vapor. The invention relates in particular to the digestion of wood chips to produce pulp from which paper can be made, and in particular to a device and method for pre-treating the wood chips that are being loaded into the digester.
One method of producing wood pulp for paper-making comprises reducing wood to chips, and digesting the wood chips to pulp (by exposing them to steam and cooking liquor) at elevated temperature and pressure. Cooking liquor is heated to approximately 350xc2x0 F. (180xc2x0 C.), and a pressure of around 150 psi (1 MPa) is maintained to insure the liquor maintains temperature. The cooking liquor is circulated throughout the digester, which dissolves the organic and non-organic material in the cellulose material. This action results in a slurry consisting of cellulose fiber. The slurry is then cleaned and dewatered, which results in a product used for making paper. It has been found that the effectiveness and efficiency of the digestion process can be improved by pre-treatment of the wood chips before they are loaded into the digester.
According to one aspect of the invention, there is provided a pre-treatment unit designed to be installed in the wood chip conveying system, between a wood chip conveyor and an inlet that is located on top of the wood chip digester, or of any vessel that is used for processing wood chips. According to another aspect of the invention, there is provided a method of pre-treating wood chips using such a unit. According to further aspects of the invention, the pre-treatment unit and method may be used with particulate materials other than wood-chips and/or for other purposes.
A pre-treatment unit according to one aspect of the invention comprises a tube having an inlet for chips at an upper end and an open lower end forming an outlet for pre-treated chips. At least one ring of injection nozzles in the wall of the tube admits into the tube flows of gas or vapor angled circumferentially to impart a rotating motion to said chips. The chips are permitted to fall freely from the inlet to and through the outlet while being exposed to the gas or vapor.
Where the pre-treatment unit is intended to pre-treat wood chips for pulping, the gas or vapor is usually steam. A suitable supply of steam at the temperatures and pressures required is then preferably provided.
Preferably, there are several rows of steam injection nozzles, grouped into steam injectors spaced apart along the length of the unit. The nozzles in the bottom row of each unit are then preferably angled slightly downwards so as to urge the chips to flow down to the next injector, while the nozzles in the top row of each subsequent injector are horizontal, to encourage the chips to dwell within the steam injector, where they are most subject to the action of the injected steam.
Preferably, there is also a row of liquor injection nozzles, spraying digestive liquor onto the chips between two of the steam injectors.
The combined action of the heat and moisture in the steam has been found not only to preheat the wood chips but also to de-aerate them, replacing the air and non-condensable vapor in the pores of the wood chips with water or digestive liquor.
Preferably, at the bottom of the pre-treatment unit, the chips are propelled down and outwards by high pressure steam. This can serve both to compact the wood chips and to spread them evenly over the plan area of the digester.
By pre-heating, de-aerating, and compacting the wood chips, it is possible to realize an increase of wood chip throughput of between 15-18 percent per batch, a reduced cooking cost, and an improvement in pulp quality.